Three-dimensional scanners are devices which enable geometrical coordinates (x, y, z) to be measured for each point of the surface of a scene or of an object. The result of the measurement is often considered as a so-called “depth” map of the scene taken as an image since the depth (or distance) z is generally expressed on the basis of the position (x, y) in a Cartesian coordinate system. The depth maps so collected may then be used to construct three-dimensional synthetic images (digital objects) for various purposes.
Several scanner technologies are used: some require a physical contact with the surface to digitize, for example coordinate measuring machines (CMMs) for which the measurement is carried out by palpating the surface by means of a probe or stylus. Others enable the measurement to be made without physical contact, for example the stereoscopic systems based on the use of two cameras, slightly spaced apart and pointing towards the same scene, for which the depth is deduced from the geometrical deformation between the two images.
So-called “structured-light” three-dimensional scanners are a specific family of contact-free three-dimensional scanners. These scanners are constituted by a light projector, generally produced on the principle of a video projector or constituted by a laser system generating interference fringes (for the so-called “phase-shifting” technique), and at least one camera that is geometrically offset relative to the projector in order to produce a stereoscopic configuration. The projector projects a so-called “structured” pattern of light, having one or two geometrical dimensions (for example a line or an image) which may possibly be colored, onto the surface to measure. The camera, positioned at a distance from the projector called “stereo basis”, acquires and records an image of the scene. The projected structured pattern is constituted by elementary patterns (called “structured elements” in this document) appropriately chosen so that it is possible to detect them in the acquired image.
The geometrical position (x, y, z) of each point of the surface of the scene observed is obtained by a triangulation method between the directions of projection and imaging of each structured element. The patterns used in the structured light three-dimensional scanners are generally projected in black and white, in grayscale, in color or in a combination of the three effects (this is for example set out in the paper by J. Geng, “Structured-Light 3D surface imaging: a tutorial”, Advances in Optics and Photonics 3, pp. 128-160, 2011). If the scene observed is static, “multiple-shot” methods of (temporally) sequential projections of patterns (for example the techniques known as “phase shifting”, “binary patterns” or “gray coding”), enable accurate and reliable measurements to be obtained thanks to the complementarity of the information available in the images of the sequence (see for example J. Geng, 2011). If the scene to observe is moving, it is necessary to use non-sequential projection methods, with one image (known as “single-shot”), or even with two images (known as “two-shot”) if the acquisition of the two images is sufficiently fast relative to the movement of the actual scene. Non-sequential techniques require projecting multiple structured elements that are sufficiently different from each other in order to be identifiable in the image acquired by the camera. These structured elements may, for example, be bands of which the colors are defined by a so-called “De Bruijn” sequence, or another possibility is coded point clouds (see J. Geng, 2011).
With the surface to digitize has high gradients of relief or discontinuities (holes or occlusions for example), the depth map produced by the structured-light three-dimensional scanner of the prior art by means of non-sequential techniques may be biased or incomplete near those singular shapes since certain structured elements projected are then only partially observable (i.e. in part concealed) or too distorted, and are not therefore always identifiable in the image acquired by the camera.
Furthermore structured-light three-dimensional scanners of the prior art essentially make it possible to produce the depth map of the scene taken as an image, or even possibly to provide information as to its appearance (color, texture, etc.), but they do not give quantitative data making it possible to deduce the nature of the materials constituting the elements of the scene.
Such information, used conjointly with the depth map, would be very useful to produce, accurately and automatically, an identification and a classification of the elements of the scene.
A method is proposed in the published patent application Nos. U.S. 2014/0028800 and U.S. 2014/0028801. It relies on a theoretical utilization of a device for projecting structured light in the spectral domain and an imaging device enabling a measurement to be made of the light reflected (or scattered) by the surface of the scene or by the object observed. The structuring of the grounds is made in the spectral domain rather than in the geometrical, colorimetric and/or temporal domains. The spectral information reflected by surface of the elements of the scene constitutes an item of data useful for determining the nature of the materials observed. It is also suggested, by way of example, to perform a multispectral projection of binary geometrical patterns, in a manner similar to the “binary patterns” technique except that the binary patterns are not projected sequentially in time but simultaneously at different wavelengths, the separation of the images ultimately being provided by the spectral imaging device. In other words, the method proposed in these patent documents relies on a hypothetical use of a projector endowed with several wavelength channels that are relatively narrow making it possible to simultaneously project different geometrical images from one wavelength channel to another. Furthermore, it is indicated that the number of channels of the projector must be quite high, typically greater than 10 by analogy with the “binary patterns” technique, to ensure sufficient accuracy in the depth map of the scene. However, no device making it possible to implement the method is described or even proposed. It is however mentioned in these documents that the wavelength channels of the projector may possibly be associated with the use of monochrome sources such as laser sources or LEDs, but the technical features enabling a prototype to be achieved are not given.
The commercially available video projectors do not enable that need to be met.
As a matter of fact, the commercially available video projectors are generally endowed with the three colorimetric channels corresponding to the colors red, green and blue (RGB), as is for example described in the published patent application Nos. U.S. Pat. No. 6,247,814B1 and U.S.2004085634A1, which is sufficient to project structured elements into the geometrical, colorimetric and temporal domains, but not to measure a spectral reflectance of the observed scene with accuracy, since this requires a wide range of wavelengths.
There are however video projectors having more than three colorimetric channels (see for example the published patent application Nos. WO2006096598A2, U.S.2010156958A1 and U.S.2010315596A1) but these projectors are bulkier and more costly, in particular on account of the fact that they comprise a more complex optical color separation prism, and that it is necessary to add a micromirror array (DMD-Digital Micromirror Device) per additional colorimetric channel.
Lastly, in order to optimize the measurement accuracy of the depth map and of the reflected spectral information, it would be desirable to be able to modify, in real time, the number and the ranges of wavelengths of the projector channels to adapt the structured light projected to the spectral characteristics of the ambient lighting and to the reflectance (that is to say the optical signature) of the elements of the scene. This last feature can only be implemented in very limited manner with a projector of which the wavelength channels are set at the time of manufacture by the selected monochrome sources.